1. Field of the Invention
The present invention relates to an antenna gain specifying device and a radio communication device, and more particularly to an antenna gain specifying device and a radio communication device capable of appropriately calculating an antenna gain.
2. Description of the Background Art
Conventionally, an apparatus for communicating signals by radio waves has an antenna attached thereto so that antenna power in transmission can be increased by an antenna gain. In reception, such an apparatus operates as if a receiver sensitivity was apparently increased by the antenna gain. As a result, the apparatus can receive a weaker signal. By making use of this phenomenon, devices that control such an apparatus in various ways have been developed.
Specifically, Japanese Patent Laying-Open No. 9-148973 discloses a diversity reception device for selecting an antenna receiving radio waves with the highest intensity by integrating or averaging multiple values representing intensities of received radio waves. Furthermore, Japanese Patent Laying-Open No. 2000-151318 discloses a peripheral level detecting device in which a time constant is changed, based on whether a peripheral level signal specified by a radio wave received by an antenna is synchronized with a radio signal, in order to improve accuracy of detecting received field intensity.
However, in the case where an antenna is selected by integrating or averaging multiple values representing intensities of received radio waves, as disclosed in Japanese Patent Laying-Open No. 9-148973, an antenna with radio waves having the actually highest intensity may not be selected. This problem arises in a radio apparatus to/from which an external antenna can be attached/removed using an external connector.
Such an apparatus can be provided with any antenna if the shape of the external connector fits. This is very convenient to apparatus-users since they can use any antenna. On the other hand, to apparatus-designers, this may result in that an arbitrary antenna, that is, an unspecific antenna having an arbitrary antenna gain will be attached.
When an arbitrary antenna is attached, it becomes impossible to develop accurate gain designs. In particular, in the case of an apparatus sensing carriers by measuring a magnitude of a signal level using an antenna terminal, a signal level appearing at the antenna terminal varies depending on a gain of an antenna as attached. Varied signal levels make it impossible to determine a signal level accurately. Therefore, a proper operation cannot be ensured.
In order to solve this problem, the apparatus having the aforementioned problem has been actually configured such that only an antenna having a known antenna gain can be used, even though an arbitrary antenna can be attached in nature. Such an apparatus is configured to operate properly according to the following method when an antenna is to be replaced. Specifically, that method includes setting the gain of the attached antenna by operating a switch or the like as separately provided.
The aforementioned problem is especially significant for an apparatus having an antenna diversity function to allow antennas to be used in a switched manner. This is because, in the structure in which a plurality of antennas can be externally attached, all the gains of a plurality of antennas to be attached may be different. Such an apparatus cannot ensure a proper operation when the antennas are switched over by the diversity function. In order to solve such a problem, an antenna gain must be individually set for each of the connected antennas. This operation is too complicated to be put into practical use.
The similar problem as described above arises in the case where a time constant is changed based on whether a peripheral level signal specified by a received radio wave is synchronized with a radio signal, as disclosed in Japanese Patent Laying-Open No. 2000-151318.